UCL Department of Space and Climate Physics Mullard Space Science Laboratory Multi- wavelength observations of active region evolution Lucie Green & Lidia van Driel- Gesztelyi
Active Region - Definition Active regions are the totality of observable phenomena in a 3D volume represented by the extension of magnetic field from the photosphere to corona, revealed by emissions over a wide range of wavelengths from radio to X- rays and γ- rays (only during flares) accompanying and following the emergence of mildly twisted magnetic flux (kg, 10 20 Mx) through the photosphere into the chromosphere and corona. The simplest ARs have bipolar magnetic field configurations, but ARs may be built- up by several bipoles emerging in close succession.
Active Region - Definition In the photosphere the presence A of NOAA strong active magnetic region field number is manifested is by the appearance of dark sunspots or assigned pores and whilst bright the faculae region representing has spots concentrated and dispersed magnetic fields, respectively. In the chromosphere arch filament systems connect opposite polarity magnetic concentrations, filaments form along the magnetic inversion line and the bright regions which appear above dispersed fields are called plages. In the transition region and corona bright, hot, dense loops connect the opposite magnetic polarities.
Active Regions & plasma beta The lower corona is a unique band in the Sun s atmosphere where the magnetic pressure dominates over the plasma pressure (plasma β= 8πp/B 2 ). Gary, 2001 Plasma β <1 up to 1.2 to 1.5 solar radii (Gary, 2001). Emerging AR magnetic field carries energy into the solar atmosphere. Emerging AR field interacts with preexisting field driving changes in topology and dynamic energy release events. 4
SDO HMI & AIA AR 12119 18-23 July 2014 evolution of a small AR of 2.5 x 10 21 Mx HMI 1600 T 10 5 + 5 10 3 304 T 5 10 4 171 T 6.3 10 5 193 T 1.2 10 6 + 2 10 7 211 T 2 10 6
Flux budget of the Sun 6
Phase I: Active region formation 7
Structure of the magnetic field Portions of the toroidal flux layer become buoyant and rise up as omega loops Early emergence suggests that flux is shredded Later emergence and formation of sunspots suggests that flux is in coherent bundle Scenario is consistent with the emergence of a fragmented omega loop Zwann, 1985 Strous & Zwann, 1999 8
Active region formation: serpentine flux emergence Cheung, LRSP (2014) The effect of granular flows on serpentine field lines (Cheung et al., 2010) Reconnection at bald patches leads to plasma heating (Ellerman bombs in Hα) Pariat et al. (2004)
Serpentine flux emergence coronal reorganisation Hinode/EIS Hinode/SOT/SP Valori et al. (2012) Emerging flux in AR 11024: NLFF analysis using the magneto- frictional method using Hinode/ SOT/SP vector data.
Serpentine magnetic field lines Valori et al. (2012) 11
Inherent twist? Electric currents computed from the gradients of the magnetic field vectors grow with the emerging flux (Leka et al., 1996) Vector magnetogram data show twist when plotted over an emerging active region Model: pitch angle: untwisted flux tubes are destroyed by vortices that form in its wake in the convection zone twist can conserve the integrity of the flux tube (Emonet & Moreno-Insertis, 1998) Location of flux tube boundary 13.9 o 7 o 2.5 o 0 o All major flux which has crossed the convection zone and emerged into the corona must be twisted? wake structures 12
Polarity patterns in emerging ARs indicate that they are twisted (Leka et al., 1996; ) flux rope partly emerged and this is observable even in the LOS field (López- Fuentes et al., 2000; Luoni et al., 2011) magnetic tongues: trace of azimuthal B new bipole Luoni, Mandrini, Démoulin, van Driel- Gesztelyi, Solar Phys. (2011) First days of an AR emergence trace of: axial B 70 Mm time => apparent rotation of the AR In MHD simulations of twisted flux emergence such tongues (or tails) are clearly present (Archontis and Hood, 2010; MacTaggart, 2011; Jouve, Brun, Aulanier, 2013; review by Hood, Archontis, McTaggart, Solar Phys., 2012).
Analytical modelling effect of twist level Luoni et al. (2011) The direction of the inversion line and the elongation of tongues depend on the level of twist (N t, the number of turns) in the half- torus model flux- rope. Using coronal extrapolations, sigmoidal shape, helicity flux for 40 ARs the sign of helicity inferred from the tongue pattern was consistent with that deduced from these other known helicity sign proxies. This simple method works AR 8757
Evolution owing to a global twist of the emerging flux tube: tongues/tails Follow- up to work by Poisson et al. (2015) on 41 bipolar ARs. Magnetic tongues/tails: analytic model for N=1 (end- to- end twist) τ is the angle between the PIL and orthogonal to the main bipole axis φ is the tilt angle The AR rotates owing to the evolution of the tongues. The total magnetic flux reaches maximum because an azimuthal component is added to the LOS axial flux. As the emergence continues the azimuthal flux contribution is decreasing and disappears, which may be interpreted as flux cancellation. Poisson, Mandrini, Démoulin, López- Fuentes, Solar Phys, 2015
Are all ARs twisted and if so, how much? Selected sample of 41 ARs 01/2003-01/2011 (isolated ARs, low background flux, emergence close to disc centre). Overall twist is relatively low, but it is evident in practically all ARs. Poisson, et al., Solar Phys, 2015 MDI HMI
Inherent twist There is a weak hemispheric preference for: negative twist in northern hemisphere positive twist in southern hemisphere (Canfield & Pevtsov, 1999) This isn t dependent on solar cycle. Q. Where do the flux tubes acquire their twist? During the dynamo generation process or through buffeting in the convection zone? Some (extreme?) cases show rotating sunspots 17
Formation of AR 12017 Use Jhelioviewer or solar monitor to look at the formation of AR 12017 Are there any tongues? what is the trend in that hemisphere? 18
Quick look at HMI line- of- sight magnetograms for AR 12017 on 23/24 March 2014 Set-up IDL to include aia & sdo paths, gunzip data if needed and extract files -> tar xvf' IDL> spawn, ls *.fits, hlist IDL> print, hlist IDL> ss=indgen(10) IDL> ss=ss*12. IDL> aia_prep, hlist(ss), -1, index, data, /uncomp_delete (see sheet about missing values) IDL> index2map, index, data, hmaps IDL> sub_map, hmaps, shmaps, xrange=[-900, -400], yrange=[100, 400] IDL> rmaps = drot_map(shmaps, ref_map=shmaps(9)) IDL> plot_map, rmaps(0), dmin=-1000, dmax=1000 IDL> movie_map, rmaps, dmin=-1000, dmax=1000, xrange=[x1, x2], yrange=[y1,y2]
Phase II: Active region decay 20
Active region evolution Démoulin et al., 2002 Largest spots may last for months Sunspots break up as flux gets pealed off the strong concentrations Magnetic structure simplifies and becomes more bipolar It is dispersed by random walk process and gathers along the supergranular boundaries The flux tube of the active region apparently gets disconnected from its toroidal roots. Lifetime: tar (days) = 15 Φ / 10 21 Mx where Φ is the total magnetic flux of the active region. Approx. 5 months for a major AR (Schrijver & Title, 1999) 21
The best AR to study long- term magneuc evoluuon: AR 7978 EFR - 1st rotation, Jul. 1996 2nd rotation 3rd rotation 4th rotation 5th rotation Q. To what extent do the active region fields remain connected to 6th the rotation subsurface fields which connect down to the (van Driel- Gesztelyi et al, tachocline? 1999, 2003; Démoulin et al, 2002, 2003, Mandrini et al., 2004, Li and Welsch, 2008, ) 7th rotation During solar minimum between cycles 22 and 23, the 4 th of a 5- AR activity nest (50% of ARs emerge in a nest). SOHO/MDI magnetograms July- December 1996
How many rotations can AR 12017 be tracked for? Rotation 1 Rotation 2
Rotation 2 Rotation 3
MagneUc area and flux density evoluuon van Driel- Gesztelyi et al., ApJ (2003) Area Flux density Mean magnetic flux density is total magnetic flux divided by the surface area of the AR.
MagneUc flux evoluuon of AR 7978 van Driel- Gesztelyi et al., ApJ (2003) Fixed area: 1.6 x 10 11 km 2 Not corrected for MDI s underestimation of flux Φ x 1.45
Flux in AR 12017 Use poly_manual_hmi.pro on (last) HMI image(s) IDL>.r wdefroi IDL> poly_manual_hmi, rmaps, posflux, negflux, pospix, negpix, movie rmaps is the input posflux, negflux, pospix, negpix, movie are all outputs 27
Milestones in the long- term evoluuon
B decrease, shearing, polarity separauon Southern hemisphere AR The PIL (NL) angle is changing in accordance with shearing by differential rotation. The weighted polarity separation hardly changes from the 3 rd to the 4 th rotation Li and Welsch (2008)
Active region evolution The long-term evolution can reflect the 3D structure of the emerging flux rope. (Lopez Fuentes et al, 2000). Green et al., 2002 30
Removal of magnetic flux Ohmic dissipation The diffusion timescale is found by equating the LHS of the induction equation with the 2 nd term on the RHS. This is the timescale of Ohmic decay due to the finite electrical resistance. For a sunspot with L=10 6 m, td ~ 30,000 years. Ohmic dissipation becomes effective on sub-granular length scales. Flux retraction Only works for very small elements where there is a large curvature such that the tension force overcomes buoyancy. Cancellation A combination of magnetic reconnection and flux retraction due to the tension force.
LifeUme funcuon of total flux AR Flux (Mx) Lifetime Emerge/Lifetime Large w. spots 5x10 weeks- months 15%- 3% Small w. pores 1x10 Days- week(s) 15%- 27% Ephemeral 3x10 Hours- 1 day ~30% Schrijver and Zwaan, 2000 The lifetime of an AR can depend, however, on its interaction with surrounding magnetic fields. The lifetime of two ARs with the same flux content can be very different depending on the phase of the cycle: a large AR may be traceable for up to 10 months during solar minimum, when it can evolve undisturbed, while its identity can be lost in less than 4 months during solar maximum (Schrijver and Harvey, 1994).
Timescales How long do ARs spend emerging? How long do ARs spend decaying? -> Most of their life spent in the decay phase
Activity evolution ARs are the principal source of a broad range of solar activity phenomena: ranging from small- scale brightenings and jets to the largest flares and coronal mass ejections (CMEs). The level and type of activity is dependent on the evolutionary stage of an AR, being highest at the emergence stage and decreasing after that.
Activity evolution in AR 7978 Yohkoh / SXT SoHO / MDI Shearing coronal loops Converging motions at PIL Flux dispersal and B decrease Flux cancellation at PIL Energetic flare rate î CME rate ~ const Démoulin et al. (2002) & van Driel Gesztelyi et al. (2003) Martin et al. (1985), Schmieder et al. (2008), Park et al. (2010), Green et al. (2011) 35
Flare and CME activity in AR 7978 15 18 11 12 Démoulin et al., A&A (2002) 36
AR 8100 37
Q. How is the CME rate maintained during the decay phase? 31 56.1 38
Finding flare and CME activity in IDL pr_gev as a quick look, but only whilst there is an active region with a NOAA label Soft X-ray light curves in the active region which can be mapped to the GOES light curves CME identification through coronagraph images and lower coronal signatures such as filament eruptions, rising EUV/SXR structures, dimming regions and EUV global waves
Formation of eruptive structures in decay phase Flux emergence Flux fragmentation and dispersal Elongation along the Y direction Flux cancellation 40
Model for flux rope formation van Ballegooijen & Martens, 1989 Filaments form during this phase 41
Formation: AR decay phase 1) Flux emergence 2) Increasing shear 3) Double-J formation 4) Continuous S Evolutionary stages in isolated bipolar regions AR 8005 ON FAR SIDE 17 hr 7 hr Feb 2007 AR 10977 ON FAR SIDE 17 hr 9 hr 12 hr 12 hr 24 hr 42
Formation: decay phase 2) Increasing shear 3) Double-J formation 4) Continuous S Evolutionary stages in isolated bipolar regions AR 8005 17 hr 7 hr Feb 2007 AR 10977 9 hr 12 hr 12 hr 24 hr 43
Flux cancellation formation of flux rope (sigmoid) Green, Kliem, Wallace (2011) 34% of the peak ve flux cancels 2.5 days prior to the eruption. If this all was built up in the flux rope, then it would contain ~60% of the AR flux. (van Ballegooijen & Martens, 1989). 44
Cancellation - flux rope formation Green, Kliem, Wallace (2011) But: cancellation can only create a flux rope from sheared fields, in fact it is re- distributing the shear, concentrating it along the PIL. The above example shows that only 2/5 cancellations were forming a flux rope out of the sheared arcade. Poloidal and axial flux in the flux ropes for most models amounts to about 60%- 70% of the cancelled flux and 30%- 50% of the total flux in the regions. (Savcheva et al., 2012) However, more cancellations more massive fluxrope? 45
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Pre-eruption structure Flux rope versus arcade Identifying flux ropes: Inverse crossing of vector field Concave up field could be produced by field lines at the bottom of a flux rope (Athay et al., 1983; Lites, 2005; Canou et al., 2009) (some?) sigmoids Inverse crossing of the PIL by S shaped field lines which survive an eruption (Fan & Gibson, 2006; Green & Kliem, 2009) so that S shaped field lines in a sheared arcade can be excluded (Antiochos et al., 1994) Plasmoids/hot flux ropes Formed and heated during magnetic reconnection. Temperature of ~10 MK (Shibata et al., 1995; Reeves & Golub, 2011; Zhang et Lites, 2005 Green & Kliem, 2009 Shibata et al., 1995 Image from Reeves & Golub, 2011 al., 2012; Patsourakos et al., 2013) 47 - +
The helicity budget in AR 7978 Démoulin et al., A&A, 2002 Mandrini et al., 2004 All values are in units of 10 42 Mx 2 Estimated helicity carried away per CME 2 x 10 42 Mx 2 The total amount of helicity which should come from twisted flux emergence and and replenished via torsional waves from the sub- surface reservoir after CMEs can be estimated to be between 55 241 x 10 42 Mx 2 in this simple bipolar region, that produced 30 CMEs. During the same period, the differential rotation injected only 8. 3 x 10 42 Mx 2, so it clearly 48 was a minor contributor to the magnetic- helicity budget of AR 7978.
Helicity evolution in two bipolar ARs The helicity source in these small bipolar active regions appears to be driven by photospheric flows 49